The use of the wind to provide power for various uses including pumping and grinding dates back to the fifteenth century in Europe. In general, windmills provided mechanical work only on a limited scale for very local use.
With the ever increasing demands on energy sources, all possible sources are being researched. This is particularly true if the new source is (i) nonpolluting and inexhaustible, (ii) capable of supplying energy on a national scale, for example, exceeding 100,000 MW in capacity (the present U.S. electrical generating capacity is around 400,000 MW), (iii) relatively simple in technology so that it can be developed in a decade, and (iv) economically feasible so that the new power plant can compete with the existing or forthcoming fossil or nuclear power plants.
Wind energy has enough potential to qualify as such a new energy source. However, its energy density is low (its kinetic energy is equivalent to 15 watts per square foot cross-section at 15 mph) and it is highly fluctuating in speed and in direction, particularly near the ground.
The challenge is then to build wind energy systems of large unit size. Each unit can collect large amounts of wind to generate many megawatts of energy, can extend to great heights (e.g. several thousand feet above the ground) where wind is more steady and abundant, and can withstand the extreme winds of hurricanes and tornadoes.
For various reasons discussed below, the standard type windmill, i.e., a propeller assembly positioned to face the wind, has failed to meet this challenge. When adapted to drive an electrical generator, a standard windmill of more than 25 feet in diameter is needed to generate sufficient power for a single home. A long-range program is being pursued by the U.S. government to build a 125 feet diameter unit for generating 100 kilowatts. And the largest unit ever built generated only 1 megawatts with blades of 200 feet in diameter.
As to the standard windmill design it has significant drawbacks which make them undesirable for small energy production and unacceptable for large energy production.
These drawbacks fall into basically three categories: fluid dynamics, stress and electrical. The fluid dynamics difficulties may be best appreciated from a consideration of the Betz momentum theory. The column of air (wind) impinging on the windmill blades is slowed down, and its boundary is an expanding envelope. Disregarding rotational and drag losses, a theoretical maximum power output, due to the slowing of the wind and corresponding expansion of the boundary envelope, is approximately 60% of the power contained in the wind. Additionally, the structure used to support the blades of the windmill and the less than ideal performance of the blades themselves present interference losses further decreasing the power output.
Mechanical stresses induced on the blading and supporting structure present a further limitation, especially for large windmills. On the supporting structure, the axial stress, representing the force tending to overturn the stationary windmill, or the thrust on the bearing must be kept within limits at all wind speeds. To accomplish these results and to generate sufficient power, large diameter blades with built-in mechanisms for adjusting the pitch angles of the blades have to be utilized. The mechanisms make the blades fragile and costly.
Large diameter blades over 100 feet in diameter present significant dynamic stress problems when used in standard windmills. The combination of gravitation force and torque force on each blade element functions to cyclically stress the element as it rotates in a rising direction and then falling direction. Moreover, with vertically rotating blades, changes in median wind velocity and the specific wind velocity at different elevation along the path of the blade greatly influence the cyclical stresses and power output of the wind turbine.
Long blades supported at their roots and under the influence of the aforementioned oscillating forces are subjected to an increasingly severe and complex system of dynamic instabilities. It becomes increasingly difficult (and expensive) to safeguard against instabilities. Blade stiffness to weight ratio improvements and advanced design methods can help, but there will always be a practical maximum to the size of a conventional wind turbine.
Finally, wind turbines of the windmill type are not well suited for use in major power installations, particularly in power grids. In order to provide stability of the network, energy generators coupled to power grids must be maintained within critical voltage and frequency ranges and must be capable of furnishing the required amount of power whenever called upon by the grid dispatcher. The standard windmill is particularly sensitive to changes in median wind speed resulting in highly variable voltage, frequency or power output produced by the generators driven thereby. Moreover, large power output required for economic operation are not feasible due to stress problems; and difficulties arise in coupling the blade shaft, which rotates in the range of 20-100 rpm, with electric generators used in power grids which operate in the range of 600-3600 rpm.